This invention generally relates to toner compositions and a dispersion comminution method of producing toners for developing latent electrostatic images in electrophotography, electrostatic recording and electrostatic printing. More specifically, this invention relates in a preferred embodiment to a dispersion comminution method of forming suitably sized resin particles which incorporate a coloring agent and other suitable components therein for high-resolution electrophotography, electrostatic recording and electrostatic printing.
The formation and development of images on the surface of photoconductive materials by electrostatic means is well known. The basic electrophotographic imaging process (U.S. Pat. No. 2,297,691) involves placing a uniform electrostatic charge on a photoconductive insulating layer known as a photoconductor or photoreceptor, exposing the photoreceptor to a light and shadow image to dissipate the charge on the areas of the photoreceptor exposed to the light, and developing the resulting electrostatic latent image by depositing on the image a finely divided electroscopic toner material. The toner will normally be attracted to those areas of the photoreceptor which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. This developed image may then be transferred to a substrate such as paper. The transferred image subsequently may be permanently affixed to the substrate by heat, pressure, a combination of heat and pressure, or other suitable fixing means such as solvent or over coating treatment.
Also well known are techniques to develop such electrostatic images. Developer is a vehicle in which are dispersed charged colored toner particles. The photoreceptor bearing the electrostatic latent image is contacted with the developer. The contact causes the charged toner particles in the developer to migrate to the charged areas of the photoreceptor to develop the latent image. Then, the photoreceptor is developed with the charged colored particles adhering to the latent image in image configuration. The developed image is then typically transferred to a suitable substrate, such as paper or transparency material, and optionally may be fixed to the substrate by heat, pressure or other suitable means.
Toners and developer compositions including colored particles are well known. In this regard, see U.S. Pat. Nos.: 5,352,521; 4,778,742; 5,470,687; 5,500,321; 5,102,761; 4,645,727; 5,437,953; 5,296,325 and 5,200,290 the disclosures of which are hereby incorporated by reference. The traditional compositions normally contain toner particles consisting of resin and colorants, wax or a polyolefin, charge control agents, flow agents and other additives. A typical toner formulation generally contains about 90-95 weight percent resin, about 2-10 weight percent colorant, from 0 to about 6 weight percent wax, from 0 to about 3 weight percent charge control agent, about 0.25-1 weight percent flow agent and from 0 to about 1 weight percent other additives. Major resins are styrene-acrylic copolymers, styrene-butadiene copolymers and polyesters. The colorants usually are selected from cyan dyes or pigments, magenta dyes or pigments, yellow dyes or pigments, and mixtures thereof.
Conventional color toner particles are produced by a milling process described, for example, in the aforementioned U.S. Pat. No. 5,102,761. In that process, a polyacrylate resin is compounded with pigments, charge control agents (xe2x80x9cCCAxe2x80x9d) and occasionally wax in a melt mixer. The resulting polymer mixture is mechanically crushed and then milled into small particles. The conventional toner particles typically have an irregular shape and a broad distribution in particle size. For optimum resolution of images and color, smaller particles perform better. Thus, for example, it is difficult to obtain resolutions better than about 600 dots/inch when the average particle size is more than about 7 xcexcm. For resolutions in the order of about 1200 dots/inch, particle sizes smaller than 5 xcexcm are typically needed. It is difficult to make particles smaller than about 7-10 xcexcm by conventional processes because of the high energy cost of producing small particles as well as uniform narrow particle size distribution.
Many previous attempts to produce small toner particles with the size smaller than 7 xcexcm have been made. For example, the aforementioned U.S. Pat. Nos. 5,352,521, 5,470,687 and 5,500,321 disclose toner particles produced by dispersion polymerization. In such a method, monomers (typically styrenic and acrylate monomers) and additives such as pigments, CCA and wax are mixed together to form a dispersion. This is then further dispersed into an aqueous or a non-aqueous medium and the monomers are polymerized to form toner particles. This method has the advantage over other methods in that spherical toner particles with a small diameter can be prepared by a single process. However, the polymerization involves a substantial volume contraction and it results in entrapment of the dispersion medium inside the toner particles. Furthermore, the polymerization is difficult to be brought to completion and a substantial portion of the monomers remains in the toner particles. The residual monomers and the entrapped dispersion solvent are difficult to separate from the particles. Also, the polarity of the polymerizing materials changes drastically in the course of the polymerization and the additives tend to exude from the particle bulk and tend to concentrate on the surface thereof. Further, agents employed, such as dispersion-stabilizing agent and surface active agent, which cause the charging characteristics and preservability of the toner particles to deteriorate, remain on the surface of the toner particles, and those agents are extremely difficult to remove from the toner particles.
A co-pending application. U.S. patent application, Ser. No. 09/571,772, discloses a method of producing toner particles by comminuting resin particles comprising a colorant and a charge control agent in a solvent which does not dissolve the resin. However, applicability of the method is somewhat limited to toner resins with a relatively low molecular weight and the method generally requires a moderately high temperature and a vigorous shearing for effective comminution of toner particles. Furthermore, the toner particles produced by the method generally have a smooth surface texture and tend to lack a fast triboelectric charging characteristics which is important in mono-component electrophotography development systems.
Another desirable property in a particulate toner composition is a narrow particle size distribution. It is generally believed that a narrow size distribution leads to a more uniform charge distribution in the toner composition which, in turn. leads to a better line resolution in a printed image as well as reduction in spotty background. The conventional milling method of producing toner particles is generally inefficient in producing particles with a narrow size distribution and therefore has to employ a classification step to remove particles that are too small or too large from the toner composition.
Narrowness of the size distribution may be expressed by the 80% span (the span). The span is defined as the ratio of the size range in which middle 80% by volume of the particles occupy to the median size. A more detailed description of the definition of the span is in a later section on the characterization methods used in the present invention. A smaller value of the span therefore means a narrower size distribution. The span value of a typical toner composition which is commercially available after afore-mentioned classification step is about 1.2. A method of toner particle formation yielding particles with the span value less than 1.2 without a classification process is highly desirable.
There is continuing interest in the development of new and improved methods of producing toners for application in high-resolution electrophotography. Some such methods have included the suggestion of dispersing polymer/solvent droplets in a water medium and shearing the mixture. However, water tends to get into the interstices between particles and agglomerate them. Once agglomeration occurs, it is very difficult to drive off the water without damaging or otherwise altering the physical properties of the particles, especially with respect to polymers having relatively low softening points, that is, below about 100xc2x0 C.
Accordingly, an object of the present invention is to provide an improved dispersion comminution method of producing high-resolution color toner which has a superior combination of properties for electrophotographic imaging systems by forming spherical toner particles with a small diameter by way of dispersing a polymer resin compounded with a colorant, a vaporizable plasticizer component and other additives in a dispersion medium including a surfactant under a vigorous shearing condition.
Another object of the present invention is to provide an improved dispersion comminution method of producing high-resolution color toner composition, wherein the comminution step may be carried out at a substantially low temperature compared to the method disclosed afore-mentioned U.S. patent application Ser. 09/571,772.
Yet another object of the present invention is to provide an improved dispersion comminution method of producing high-resolution toner, wherein a polymer resin with a substantially high molecular weight may be expeditiously comminuted.
Still another object of the present invention is to provide a method of producing toner particles comprising a polymer resin, a colorant and optionally a charge control agent, which particles are substantially spherical in shape with a diameter in the range of about 1 to 10 xcexcm as well as a narrow particle size distribution.
A still yet further object of the present invention is to provide a particulate toner composition of particles which are substantially spherical in shape and have a serrated surface texture.
Yet another object of the present invention is to provide a toner composition which exhibits a fast triboelectric charging behavior and therefore are suitable for applications in a mono-component electrophotography development system.
Other objects and advantages of the present invention shall become apparent from the accompanying description and examples.
There is provided in accordance with the present invention particulate toner resin particles containing a resin component, a colorant component, and optionally a charge control agent characterized in that the toner resin particles have a micro-serrated surface exhibiting a surface roughness index of greater than about 1.2. Roughness indices of greater than about 1.5 or 2 are believed readily achieved if so desired.
In another aspect of the present invention there is provided a process for preparing a particulate toner composition for developing latent electrostatic images including the steps: a) preparing a first resin composition containing a resin component, one or more colorant components, optionally a charge control agent, other additives such as wax and fumed silica and a vaporizable plasticizer component which reduces the melt viscosity of the resin composition and thereby facilitates the overall comminution process of this invention; b) dispersing the resin composition in an organic medium comprising a surfactant, wherein the resin component is substantially insoluble in the organic medium; c) comminuting the resin composition to form particulate resin particles by an application of shear at an elevated temperature; d) removing the vaporizable plasticizer component by evaporation by maintaining the dispersion of particulate toner composition in the medium at a second elevated temperature; e) recovering the toner particles using a filtration process, followed by washing with an organic solvent with a low boiling temperature and subsequently drying the particles. Optionally, fumed silica particles are blended with the toner particles to improve flow characteristics of the toner particles. Without intending to be bound by any theory, it is believed the micro-serrated structure of the toner particles is imparted to them during removal of the vaporizable plasticizer.
In a preferred aspect, the particulate toner composition comprises a polymer resin, one or more colorants that may be pigments, dyes or combinations thereof, an optional charge control agent and other additives commonly used in the preparation of a toner composition such as wax, fumed silica particles, etc. The toner particles are substantially spherical in shape and have a volume average diameter in the range of from about 1 to about 10 microns. Furthermore, the toner particles have a uniform and narrow size distribution with the span value less than 1.0, more preferably, with the span value less than 0.8. A particularly desirable and surprising aspect of the present invention is that the toner particles may be made to have an irregular surface texture that increases the surface area and thus substantially improves the triboelectric charging characteristics of the toner composition such as the charging speed. A fast triboelectric charging characteristics of a toner composition is particularly important when the toner composition is used in a mono-component development systems which are widely employed in desktop laser printers.
Any suitable polymer resin may be employed as the resin component of the present invention. Particularly preferred resins include polyester resins and styrene copolymer resins. The polymer resin is typically an amorphous resin with a glass transition temperature in the range of from about 40xc2x0 C. to about 90xc2x0 C. The use of a vaporizable plasticizer component in the present method of producing toner particles significantly increases the molecular weight range of polymer resin usable for toner application. A desirable molecular weight range of a polymer resin processable with the method of the present invention is a weight average molecular weight in the range of from about 3000 g/mol to about 100,000 g/mol. The resin may optionally contain functional moieties which improve the compatibility with colorants as a part of its polymer chain chemical structure.
The first resin composition is typically prepared by melt mixing the resin component with a colorant, optionally with a charge control agent and other additives, and also with the vaporizable plasticizer component. The colorant is typically selected from cyan pigments or dyes, yellow pigments or dyes, magenta pigment or dyes and black pigments or dyes. The charge control agent may be dispersed in the resin and may be a positive charge control agent or a negative charge control agent. Other additives commonly used in a toner composition such as wax and famed silica particles may be dispersed.
Presence of the vaporizable plasticizer component significantly reduces the flow temperature of the first resin composition and therefore allows the whole toner preparation process to be carried out at a substantially lower temperature than the process without a vaporizable plasticizer component. The vaporizable plasticizer component may be present in any suitable amount. In general, vaporizable plasticizer in an amount between about 1 to about 200 weight percent of the resin component in the melt mixture is employed; with from about 5 to about 100 weight percent of the resin component being typical. From about 5 to 50 weight percent or 10 to 30 weight percent of the resin component may, for example, be preferred in some embodiments. The vaporizable plasticizer component is selected from organic solvents which are absorbable in the polymer resin component and have a boiling temperature less than 200xc2x0 C. It is preferable that the vaporizable plasticizer component is insoluble in the organic medium employed in the dispersion preparation and comminution steps of the present invention. Typical vaporizable plasticizers may include acetone, 1,2-dichlorethane, tetrahydrofuran, acetonitrile, 1 -methyl-2-pyrrolididone, methylethylketone, 3-pentanone, chlorobenzene, N,N-dimethylformamide, cyclohexanone, and dimethylsulfoxide. Preferred examples of the vaporizable plasticizer components are acetone, dimethylformamide, cyclohexanone, dimethylsulfoxide, and chlorobenzene. The first resin composition may be prepared by melt compounding at a temperature which is determined, in part, by the choice and the amount of vaporizable plasticizer component in the first resin composition. It is preferable to carry out the preparation of the first resin composition at as low a temperature as is practical.
The first resin composition is dispersed in the immiscible organic medium by subjecting the mixture of the molten resin composition and the organic medium to a shear at an elevated temperature. Any suitable mixing equipment may be employed for this step. An example of such equipment is a vessel equipped with an impeller-type agitator and a means of heating the content of the vessel. Effective formation of dispersion as well as successful comminution requires that the solubility parameter of the organic medium be generally different from the solubility parameter of the resin component by at least about 1. In preferred embodiments the solubility parameter of the organic medium is larger or smaller than the solubility parameter of the resin component by at least about 2. Any suitable organic medium which does not dissolve the resin component may be employed. Particularly preferred solvents include paraffin solvents and poly (ethylene glycol).
The organic medium typically includes a surfactant which may a non-ionic, a cationic or an anionic surfactant. Preferred examples of such surfactants include copolymers of vinylpyrrolidonone, alkylated maleic acid copolymers, polymers containing ethylene oxide moieties, polymers containing propylene oxide moieties and sodium dodecylsulfate. The surfactant is generally present in the organic medium in an amount from about 0.2 to about 15 weight percent based on the amount of solvent present whereas from about 1 to about 10 weight percent based on the amount of solvent present is typical.
The first resin composition is generally from about 10 to about 70 weight percent of the combined weight of the resin composition in the organic medium during the step of dispersing the first resin composition. From about 20 to about 50 weight percent of the combined weight of the first resin composition in the organic medium is more typical. The first resin composition may be introduced to the organic medium maintained at an elevated temperature under a shearing condition. Equally preferably, the organic medium may be introduced to molten first resin composition maintained at an elevated temperature under a shearing condition. During the step of dispersing the first resin composition, the organic medium is maintained at an elevated temperature. The temperature may be selected to be any value so long as it is high enough to ensure fluid-like behavior of the first resin composition and low enough not to have a substantial evaporation of the vaporizable plasticizer component in the first resin composition. Therefore the temperature may be selected to be any value by varying the type and the amount of the vaporizable plasticizer component in the first resin composition. While any suitable elevated temperature may be employed, preferred temperatures are in the range at least about 30xc2x0 C. to about 200xc2x0 C.
The step of comminuting the first resin particles is typically carried out by further subjecting the dispersion of the first resin composition in the organic medium at an elevated temperature. The comminuting temperature may be selected to be any value so long as it is high enough to ensure fluid-like behavior of the first resin composition and low enough not to have a substantial evaporation of the vaporizable plasticizer component. Therefore the temperature may be selected to be any value by varying the type and the amount of the vaporizable plasticizer component in the first resin composition. While any suitable elevated temperature may be employed, preferred temperatures are in the range at least about 30xc2x0 C. to about 200xc2x0 C. However, it needs not be the same temperature as the dispersion temperature. The shearing required for the comminuting step of the present invention is substantially smaller due to the presence of the vaporizable plasticizer component compared to that for the process without a vaporizable plasticizer component. Effective comminution may be obtained in a vessel containing a 10 cm radius impeller-type agitator and with the agitator rotation speed as low as 100 rpm.
The step of removing the vaporizable plasticizer component from the comminuted first resin composition is typically carried out by maintaining the mixture of the first resin components and the organic medium at a second elevated temperature close to or above the boiling temperature of the vaporizable plasticizer component. Under the condition, the vaporizable plasticizer component evaporates from the comminuted particulate resin composition and subsequently from the processing vessel. The process may be more expeditiously carried out when the vaporizable plasticizer composition is immiscible with the organic medium. The removal step is stopped when the vaporous effluent from the process vessel does not show a trace of the vaporizable plasticizer component.
The steps of dispersion, comminution and removal of the vaporizable plasticizer component may be conducted in distinctive and discontinuous steps, sequentially in a single vessel or in an overlapping manner in a single vessel.
The step of recovering the comminuted toner particles is carried out by first cooling the content of the process vessel below the glass transition temperature of the resin component and subsequently by filtering solid toner particles from the organic medium. Any suitable filtration equipment may be used. Subsequently, dry toner particles are obtained by washing the filtered particles with a low boiling organic solvent such as isohexane and drying off the wash solvent at a temperature below the glass transition temperature of the resin component.
A flow improvement agent, such as fumed silica particles, may be added to the toner composition, typically after the particles have been comminuted.
In another aspect of the present invention, there is provided a particulate toner composition including toner particles that are substantially spherical in shape, have an average diameter of from about 1 to about 10 microns, and have a uniform and narrow size distribution with the span value less than 1.0, more preferably, with the span value less than 0.8, prepared by comminuting a precursor composition comprising a vaporizable plasticizer component in an organic medium under shear at an elevated temperature wherein the particles are substantially insoluble in the organic medium. The toner resin may be a polyester resin or a styrene copolymer resin. The developer compositions may further include carrier particles. Such particles are typically selected from the group consisting of ferrite, steel, iron powder, and mixtures thereof, wherein the powders have a surface active agent coated thereon.
In yet another aspect of the present invention, there is provided a particulate toner composition comprising toner particles that are substantially spherical in shape, have an average diameter of from about 1 to about 10 microns, have a uniform and narrow size distribution with the span value less than 1.0, more preferably, with the span value less than 0.8 and further have an irregular surface texture characterized by the surface roughness index greater than 1.2, the surface roughness index being defined as the ratio of surface areas of the irregular textured particles and smooth texture particles. The toner resin may be a polyester resin or a styrene copolymer resin. The developer compositions may further include carrier particles. Such particles are typically selected from the group consisting of ferrite, steel, iron powder, and mixtures thereof, wherein the powders have a surface active agent coated thereon.
In still yet another aspect of the present invention, there is provided a particulate toner composition comprising a polyester resin component and a colorant component wherein the particles are substantially spherical in shape, have a volume average diameter in the range of from about 1 to about 10 microns, have a uniform and narrow size distribution with the span value less than 1.0, more preferably, with the span value less than 0.8 and have an irregular surface texture characterized by the surface roughness index greater than 1.2 wherein the polyester resin component includes a polyester resin having a weight average molecular weight of about 100,000 g/mol or less.
In general, it may be possible to achieve surface roughness indices of greater than 1.2 or so and up to as high as 5 or more and span values of the particle size distribution of less than 0.8 down to 0.5 or even 0.2.
In a still further aspect of the present invention, there is provided a particulate toner composition comprising a styrene copolymer resin component and a colorant component wherein the particles are substantially spherical in shape, have a volume average diameter in the range of from about 1 to about 10 microns, have a uniform and narrow size distribution with the span value less than 1.0, more preferably, with the span value less than 0.8 and have an irregular surface texture characterized by the surface roughness index greater than 1.2 wherein the styrene copolymer resin component includes a styrene copolymer having a weight average molecular weight of about 100,000 g/mol or less. Particularly preferred styrene copolymer resins include copolymers of styrene and acrylate as well as copolymers of styrene and butadiene.